Usually, a hybrid circuit for use in a telephone transmission system and the like employs a hybrid transformer, which is used to transfer onto a two-wire line side a signal arriving from a four-wire receiving line and to transfer a signal from the two-wire line to a four-wire transmitting line. In a hybrid circuit of this type, in order to prevent the four-wire input signal from leaking to the four-wire transmitting line, a balancing network is used which has an impedance equal to the impedance of the two-wire line side as viewed from the hybrid transformer. The impedance of the two-wire line side, however, changes or fluctuates depending upon the type and length of the line, the type of telephone set connected at the terminal end of the line, etc. In addition, the impedance of the line varies depending upon the frequency of the transmission. It is therefore difficult to realize a circuit which is applicable to many types of two-wire lines.
As a solution to this problem, it has been proposed to juxtapose a plurality of hybrid circuits having balancing networks and to select the optimum one. This measure, however, has not been put into practical use because it results in a large-sized apparatus having a high cost of manufacture.